1. Field of the Invention
The present invention relates to a polarizing filter and a producing method thereof, and more particularly to a polarizing filter and a producing method thereof, which is used in an electrical display, such as a liquid crystal panel and the like.
2. Description Related to the Prior Art
An iodine type of a polarized film is most widely used in an electrical display. In order to produce the polarized film of this type, a polymer film whose main content is polyvinylalcohol series are dipped in an iodine-potassium iodide solution, and thereafter, the stretching and the orienting of the polymer film are performed. As the polarized film of the iodine type loses the polarizing characteristics in contact with water or by being heated, a protective film is usually provided on each surface of the polarized film. Substantially, the polarizing filter is constructed of a polarized film and the protective films provided on both sides thereof.
Formerly, the same protective films were adhered to both surface of the polarized film. Concretely, the protective films were formed of the same polymers to have the same thickness and the same physical properties. Recently, however, characteristics for optical compensation, birefringence, control thereof, and antiglare are required to the protective films. Accordingly, the protective films on the both surface of the polarized film have different thickness and different physical properties each other.
As the protective film, cellulose acylate film, such as cellulose acetate film and the like, is widely used in view of low birefringence, transparency, and easy handling. Cellulose acylate film has an excellent transparency, an adequate moisture permeability, and a large mechanical strength. Further, the polymer film of the polyvinylalcohol type is widely used as the polarized film.
The above cellulose acylate film is produced in a method of producing a film from a dope solution, which polymer is dissolved to or dispersed in a solvent to prepare. The dope solution is cast on a substrate with a die, dried and, if necessary, cooled to form the polymer film. Then the polymer film is peeled from the substrate and fed with a feeding means to dry the polymer film furthermore. Note that as the transporting means there are a feed roller which applies a tension force in a feeding direction, and a tenter drier (or tenter) which tenses the polymer film in a widthwise direction. Further, the feed rollers and the tenter drier may be combined to feed the polymer film.
However, when the different protective films are adhered to the both surfaces of the polarized film, then the polarizing filter forms a curl. Accordingly, the appearance of the polarizing filter becomes bad and the attachment of the polarizing filter to a liquid crystal panel becomes harder.